1. Field of the Invention
The present invention relates to an optical scanning device and an image forming apparatus.
2. Description of the Related Art
Among image forming apparatuses, such as copiers, printers, and facsimile machines, some are known that radiate a latent image carrier through deflection scanning with a writing beam according to image information to form a latent image on the latent image carrier and develop the latent image to obtain an image. The optical scanning device for deflection scanning with a writing beam generally includes optical components, such as an imaging lens for forming an image with a writing beam deflected and scanned by a polygon scanner as a deflection scanning unit with polygon mirrors as rotating multifaceted mirrors for deflecting and scanning a writing beam from a light source. These components are housed in a housing, and the housing is covered with a covering member so that dust is not attached to the optical components, such as the imaging lens.
The polygon scanner includes a polygon mirror, a polygon motor that drives the polygon mirror for rotation, and a circuit board having mounted thereon electronic control components for controlling the driving of the polygon motor.
When a writing beam is deflected, this polygon motor rotates at a high speed equal to or higher than 30000 revolutions per minute (rpm), thereby producing heat at the bearing portion of the polygon motor. Also, at the time of driving the polygon motor, the electronic control components on the electronic control components produce heat. Since the optical scanning device is enclosed by the covering member, heat produced at these heat producing portions of the polygon motor (the bearing portion and electronic control components) is accumulated inside the optical scanning device and raises the inside temperature. When the temperature inside the optical scanning device is increased, the optical components, such as the imaging lens, may be deformed by heat. If the optical components are deformed by heat, a writing beam may not be able to form an image on the surface of the latent image carrier. Also, in the case of a color image forming apparatus, each radiation potion on the surface of each latent image carrier is shifted to cause a color shift.
Japanese Patent Application Laid-Open No. 2001-242408 discloses a conventional optical scanning device as explained below. That is, a heat dissipating unit is fixed to a lower case of a polygon scanner unit in which only the polygon scanner is covered by an upper case and the lower case. The body of the optical scanning device on which optical elements and the like are mounted has a convex groove portion for introduction of outside air. The heat dissipating unit protrudes from the bottom surface of this groove portion to mount the polygon scanner unit on the body. An open side of the groove portion of the body is covered to form a duct that is rectangular in cross section. From one side of this duct, outside air is caused to flow into the duct with a fan and others to cool the dissipating unit. With such a configuration of the optical scanning device, heat of the polygon scanner unit is transmitted to the heat dissipating unit via the lower case and is released to outside the device. As a result, an increase in temperature inside the optical scanning device can be suppressed, thereby suppressing heat deformation of the optical components.
However, the conventional optical scanning device has a problem in that the polygon scanner unit enclosed by the lower case and the upper case is mounted on the body, which increases the number of components by the lower case and the upper case, resulting in an increase in the price of the device.